AGPLv3 Free SoftwarePySoy is a Copyleft 3D game engine for Python made available under the GNU Affero General Public License (AGPL), an  OSI-approved Open Source license.

So why not call it "Open Source"?

We avoid using the term Open Source due to it's ambiguous meaning in the freedoms protected for players.

Open Source is often used as a synonym for the term Free Software, but many proprietary software vendors misuse the label Open Source to market their products which only include limited rights. The  Open Source Initiative (OSI) has a poor track record for enforcing their trademark.

Further, most Open Source 3D game engines are not Copyleft. The BSD license does not protect players' freedoms at all, and the GNU LGPL license only protects their freedom to use, modify, and redistribute the game engine itself. Neither of these licenses protect the liberty of players to use, modify, and redistribute games using the engine.

We therefore recommend refering to PySoy and games based on it as Copyleft instead of Open Source.